CO is a gas that is slightly lighter than air, so it rises.. Smoke rises because of the heat that produced it and depending on particulate density will settle. But in the incipient phase will always rise. CO detectors are time weighted. So it is possible to get false alarms even when there are less that 35ppm. (35ppm is the allowable work place exposure over an 8 hr day.) So, since a home is different than a boat, let's explore. NFPA requires smoke detectors OUTSIDE bedrooms. Why? So that it goes into alarm mode while you are safely behind a closed door. Smokes in your house are best mounted on the ceiling. Same on a boat. Smoke should be on the overhead. Because most overheads are curved, the smokes need to be mounted centerline. Now, where? I would argue as close to the main companion way as possible. Why? Well, if you are sleeping in the v-berth with the hatch opened for air, and your boat is into the wind, the incoming air would drive smoke out the main companion way and trip the detector. and because you have a combo gas detector, you've covered the other base.
BUT, don't stop here. I used to teach EDITH.... exit drill in the home.. Same on your boat. Have a PLAN ! If you are awakened by an alarm and either a haze or smoke condition exists... GET OUT ! When you go to bed, keeps PFDs and a hand held in an readily available place.